1. Field of the Invention
This invention relates to a method of separating (splitting) a composite member and a process for producing a thin film. Particularly, this invention belongs to a technical field of processes preferable for producing thin films used in solar cells and semiconductor wafers.
2. Related Background Art
Formation of single-crystal semiconductor layers on insulators is called SOI, i.e., silicon-on-insulator or semiconductor-on-insulator. A large number of researches have been made thereon because the devices utilizing the SOI technique have numerous superior points that can not be achieved by usual bulk silicon substrates on which silicon integrated circuits are to be fabricated. Specifically, the utilization of SOI technique brings about the following advantages, that is, the SOI technique is superior in, e.g., the following points.
(1) It enables high integration with easy separation of dielectrics.
(2) It promises a superior radiation resistance.
(3) Stray capacity can be reduced to enable high-speed performance.
(4) The well step can be omitted.
(5) Latch-up can be prevented.
(6) Fully depleted type electric-field effect transistors can be accomplished by thin-film formation.
Among processes for producing SOI silicon wafers, Japanese Patent Application Laid-open No. 5-21338 or U.S. Pat. No. 5,371,037 discloses a process for forming a non-single-crystal semiconductor layer on a porous layer and transferring this semiconductor layer to a supporting substrate via an insulating layer, which is very superior in view of the advantages such that SOI layers have good layer-thickness uniformity, the crystal defect density of SOI layers can be controlled to a low level with ease, SOI layers have a good surface smoothness, any specially designed expensive apparatus are not required in its production, and SOI having a layer thickness ranging widely from several nm to about 10 xcexcm can be produced using the same apparatus.
The process disclosed in Japanese Patent Application Laid-open No. 7-302889 or U.S. Pat. No. 5,856,229 is a process for forming a non-porous single-crystal semiconductor layer on a porous layer formed on a first substrate, bonding the non-porous single-crystal layer to a second substrate, and thereafter separating the first substrate from the second substrate at the porous layer without breaking the first and second substrates. Then the surface of the first substrate is smoothed, and a porous layer is again formed thereon, whereby the first substrate can be reused a plurality of times. Thus, the production cost can greatly be reduced and also the production process itself can be simplified.
As methods of separating a composite member such as bonded substrates without breaking both the first substrate and the second substrate, available are a method of pulling the composite member in the direction perpendicular to the bonded surface, a method of applying a shear stress in parallel to the bonded surface (e.g., a method of moving the respective substrates in the direction opposite to each other at the plane parallel to the bonded surface, or a method of turning the respective substrates in the opposite directions), a method of applying a pressure in the direction perpendicular to the bonded surface, a method of applying a vibratory energy such as ultrasonic waves to the separation layer, a method of inserting a separating member (e.g., a sharp blade such as a knife) to the separation layer from the side surface of the bonded substrates (composite member) in parallel to the bonded surface, a method of utilizing an expansion energy of a substance made to soak into the porous layer that functions as a separation layer, a method of thermally oxidizing the porous layer that functions as a separation layer from the side surface of the composite member to cause the volume expansion of the porous layer, thereby separating the composite member, a method of selectively etching the porous layer that functions as a separation layer from the side surface of the bonded substrates (composite member) to separate the composite member, and a method of using a layer capable of producing microcavities which is formed by ion implantation as the separation layer and heating by irradiation with laser light to separate the composite member.
To describe specifically, the surface of a non-porous substrate such as a silicon wafer is anodized at two stages while changing the concentration of hydrofluoric acid of an anodizing solution or changing anodization currents to form a plurality of porous layers. The porous layers are subjected to heat treatment at 400xc2x0 C. for 1 hour in a dry atmosphere of oxygen to form a silicon oxide layer of about 1 nm thick at the pore surfaces of the porous layers. On the layer surface of the porous layer, epitaxial growth is carried out by CVD (chemical vapor deposition) to form a non-porous layer. The surface of the non-porous layer is oxidized, and thereafter the non-porous layer is bonded to a silicon wafer prepared separately. To separate the resulting bonded wafers (composite member) into two parts, a wedge is inserted to the side surface of the bonded wafer, whereupon the porous layer having a relatively low mechanical strength is cracked to separate the bonded wafers.
In the separation of the composite member, however, if the separation layer has a high mechanical strength, the composite member tends to break before the composite member is completely separated. On the other hand, if the separation layer has a low mechanical strength, particles tend to occur.
Even if it has been able to form a separation layer having an appropriate mechanical strength, the separation layer may have no reproducibility for a position of the separation layer to be cracked when separated. In such a case, particles tend to occur, or conditions for the subsequent treatment of the separated member or thin films can not be uniformed, resulting in, e.g., an increase in production cost.
A first object of the present invention is to provide a method of separating (splitting) a composite member by which the separation layer can be cracked always at a fixed position, and a process for producing a thin film by such a method.
A second object of the present invention is to provide a method of separating a composite member by which the probability of break and the rate of occurrence of particles can be reduced and the conditions for treatment after separation can be set with ease, and a process for producing a thin film by such a method.
The present invention provides a method of separating a composite member, comprising the steps of;
forming a separation layer inside a composite member;
forming inside the separation layer a stress riser layer in which an in-plane stress is concentratedly generated to an extent that does not cause separation by itself; and
enlarging the in-plane stress to crack the stress riser layer.
The present invention also provides a method of separating a composite member, comprising the steps of;
forming at the surface of a first substrate a porous layer having a thickness of at least 1 xcexcm;
thereafter oxidizing pore inner-wall surfaces of the porous layer to form oxide films;
removing the oxide films formed at the pore inner-wall surfaces, in a depth of at least 1 xcexcm from the layer surface of the porous layer so as to leave the oxide films at the lower part of the porous layer;
forming a non-porous layer on the surface of the porous layer;
bonding the non-porous layer to a second substrate to form a composite member having a separation layer internally; and
causing a crack in the separation layer to separate the composite member.
The present invention still also provides a method of separating a composite member, comprising the steps of;
forming on the surface of a first substrate a first porous layer and a second porous layer having a higher porosity than that of the first porous layer;
oxidizing the second porous layer to make the second porous layer amorphous;
forming a non-porous layer on the layer surface of the first porous layer;
bonding the non-porous layer to a second substrate to form a composite member having a separation layer internally; and
causing a crack in the separation layer to separate the composite member.
The present invention further provides a method of separating a composite member, comprising the steps of;
generating a temperature difference in a composite member having a separation layer internally; and
causing a crack in the separation layer to separate to the composite member.
The present invention still further provides a method of separating a composite member, comprising the steps of;
preparing a composite member having a separation layer internally;
bending the composite member; and
causing a crack in the separation layer to separate the composite member.
The present invention still further provides a process for producing a thin film by any one of the separation methods described above.
The present invention is based on a finding that, as long as a stress can be concentrated at a fixed position in the separation layer, the separation proceeds along the position where the stress has been concentrated. For example, layers containing microcavities, e.g., two porous layers having porosities different from each other are formed as the separation layer. Then, a predetermined in-plane stress is concentratedly generated at the interface between the two porous layers to form a stress riser layer (stress concentrated layer). Thereafter, a separation energy is externally applied in such a way that the in-plane stress increases until it exceeds the yield stress (a stress high enough for the separation layer to be cracked), whereupon a crack or a rupture is caused along the interface (i.e., at the stress riser layer in a higher porosity porous layer). Thus, the position to be cracked in the separation layer can be made stable, and hence the thickness of the remaining separation layer can be made always uniform and any unevenness of the exposed surface (separation surface) can be made to have been controlled. Accordingly, in the subsequent steps, various treatments can be made under the same treating conditions, so that the particles can be made to greatly less occur.
The separation method of the present invention, embodied in several ways, can provide a composite member separation method which locates a crack in a restricted portion of a separation layer, and a process for producing a thin film by such a method.